1. Field of the Invention
The present invention relates to a surface light source device using point light sources such as light-emitting diodes and a liquid crystal display device using such a surface light source device.
2. Description of the Related Art
Various kinds of flat display devices have been proposed and commercialized that include devices utilizing the birefringence of a liquid crystal, devices using electroluminescence or plasma light emission, and devices in which minute electron guns or optical reflectors are arrayed in the same number as the number of pixels. Among those kinds of flat display devices, at present, the device using a liquid crystal is put in practice use widely.
In general, a liquid crystal display device is composed of a liquid crystal panel in which a liquid crystal is held between two substrates that are opposed to each other, circuit units that drives the liquid crystal panel, an illumination device for illuminating the liquid crystal panel in the display area with uniform light, and structural members that keep the liquid crystal panel, the circuit units, and the illumination device in a prescribed positional relationship. Liquid crystal display devices for certain purposes are not provided with any illumination devices but use external light (reflection type).
Illumination devices for a liquid crystal display device are classified into two types, that is, a front light in which the liquid crystal panel is illuminated from its front side and a backlight in which the liquid crystal panel is illuminated from its back side. A backlight is used in large-size liquid crystal devices of 10 inches or more, and conventionally a cold cathode fluorescent tube is used as its light source.
FIG. 9 shows a conventional backlight using a cold cathode fluorescent tube. A light guide plate 1 made of a transparent material has a pair of major surfaces 1a and 1b that are opposed to each other. The one major surface 1a is made a light-emitting surface (the major surface 1a will be hereinafter referred to as xe2x80x9clight-emitting surface 1axe2x80x9d) and the other major surface 1b is provided with a reflection sheet 4 that is a light reflecting means. At least one side surface (in FIG. 9, a side surface 1c) of the light guide plate 1 serves as a long and narrow light incidence surface (the side surface 1c will be hereinafter referred to as xe2x80x9clight incidence surface 1cxe2x80x9d). At least one lamp (cold cathode fluorescent tube) 10 is disposed close to the light incidence surface 1c. Although not shown in FIG. 9, a plurality of optical sheets for effective use of light are disposed above the light-emitting surface 1a and a reflection member for gathering light into the light incidence surface 1c is disposed around the lamp 10. The light guide plate 1 is mostly made of PMMA (poly methyl methacrylate), PC (polycarbonate), or glass each of which has large light transmittance.
Light that is emitted from the lamp 10 enters the light guide plate 1 through the light incidence surface 1c, travels through the light guide plate 1, and is output uniformly from the light-emitting surface 1a by virtue of a print formed on the light guide plate 1 and other means.
However, cold cathode fluorescent tubes used in conventional backlights employ mercury and hence have difficulty in taking proper measures against recent environmental problems. Cold cathode fluorescent tubes have another problem that the luminance lowers as the mercury is consumed.
On the other hand, in recent years, backlights and front lights using LEDs (light-emitting diodes) instead of a cold cathode fluorescent tube have been developed for small-size liquid crystal devices for cellular phones. Small-size liquid crystal devices that require only several LEDs mainly use expensive white LEDs. However, it is difficult for large-size liquid crystal display devices that require a lot of LEDs to use expensive white LEDs; at present, they cannot use any other light sources than a cold cathode fluorescent tube. To use LEDs for a large-size liquid crystal display device with a low cost, it is necessary to combine monochrome LEDs (e.g., red, green, and blue LEDs) to obtain white light instead of using white LEDs.
FIG. 10 illustrates a problem that arises when LEDs are used instead of the lamp 10 in a conventional large-size backlight. Components in FIG. 10 having the same or corresponding components in FIG. 9 are given the same reference symbols as the latter.
As shown in FIG. 10, where an LED board 2 having a plurality of LEDs 3 is disposed close to the light incidence surface 1c of the light guide plate 1, since the LEDs 3 are point light sources, the quantity of light that is incident on the light incidence surface 1c increases as the position comes closer to each LED 3 and only portions of the light-emitting surface 1a that are close to the respective LEDs 3 are bright. Therefore, where LEDs 3 of a plurality of colors are used, chrominance unevenness occurs in portions of the light-emitting surface 1a that are close to the LEDs 3 in such a manner each portion assumes a color corresponding to the color of the closest LED 3. Also where white LEDs 3 are used, similarly, only portions of the light-emitting surface 1a that are close to the respective LEDs 3 are bright, which means luminance unevenness. In FIG. 10, reference numeral 11 denotes such chrominance or luminance unevenness. Such chrominance or luminance unevenness 11 is a problem that is fatal to a backlight for a liquid crystal display device.
A simplest, effective method for solving the chrominance or luminance unevenness 11 is to increase the distance between the LED board 2 and the light incidence surface 1c of the light guide plate 1. However, recent liquid crystal display devices have been reduced in frame width and the distance between the light incidence surface 1c and the corresponding outer surface of the liquid crystal display device is now as short as about 10 mm. It is therefore difficult to increase the distance between the LED board 2 and the light incidence surface 1c. There is another problem that designing and manufacturing different LED boards 2 for individual sizes and types of liquid crystal display devices is costly and hence makes it difficult to produce inexpensive liquid crystal display devices.
The present invention has been made to solve the above problems, and an object of the invention is therefore to provide an inexpensive surface light source device that does not cause chrominance or luminance unevenness, can accommodate frame width reduction of a liquid crystal display device, and is environment-friendly, as well as an inexpensive liquid crystal display device that has superior display characteristics by virtue of the use of such a surface light source device.
A surface light source device according to the invention comprises a thin rectangular light guide plate, a light source assembly and a prism member. The thin rectangular light guide plate has a pair of major surfaces that are opposed to each other and a plurality of side surfaces that connect the pair of major surface. One of the major surfaces forms a light-emitting surface and the other major surface is provided with light reflecting means. One of the side surfaces forms a light incident surface. The light source assembly includes a plurality of point light sources so as to be arranged along a longitudinal direction of the light incidence surface of the light guide plate. The prism member is disposed between the light source assembly and the light incidence surface. The prism member includes a prism sheet consisting of a plurality of minute prisms that are arranged along an arrangement direction of the point light sources and that have a function of refracting light that is emitted from each of the point light sources and thereby inputting, to the light incidence surface, refracted light that is scattered to both sides in the longitudinal direction of the light incidence surface. The prism sheet is opposed to the point light sources and the light incidence surface with air layers interposed in between, respectively.
In this surface light source device according to the invention, since light beams emitted from the point light sources are scattered effectively, chrominance or luminance unevenness can be prevented even if the distance between the point light sources and the light incidence surface is short. Therefore, the invention can provide an inexpensive surface light source device capable of accommodating frame size reduction of a display device.
A surface light source device according to another aspect of the invention includes a thin rectangular light guide plate, a light source assembly and a light diffusion material. The thin rectangular light guide plate has a pair of major surfaces that are opposed to each other and four side surfaces that connect the pair of major surfaces. One of the major surfaces forms a light-emitting surface and the other major surface is provided with light reflecting means. One of the side surfaces forms a light incident surface. The light source assembly includes a plurality of point light sources that are disposed close to the light incident surface of the light guide plate so as to be arranged along a longitudinal direction of the light incidence surface. The light diffusing material is mixed in a light incidence portion of the light guide plate that is close to the light incidence surface.
In this surface light source device, the light diffusing material is mixed in a light incidence portion of the light guide plate that is close to the light incidence surface. By mixing the light diffusion material into a light incidence portion of the light guide plate that is close to the light incidence surface, light that has reached the light incidence surface after being refracted by the prism element and thereby scattered to both sides in the longitudinal direction of the light incidence surface can further be diffused in the portion close to the light incidence surface. This enhances the effect of preventing chrominance or luminance unevenness. Since light beams emitted from the point light sources are scattered effectively, chrominance or luminance unevenness can be prevented even if the distance between the point light sources and the light incidence surface is short. Therefore, the invention can provide an inexpensive surface light source device capable of accommodating frame size reduction of a display device.
The invention also provides a liquid crystal display device includes the surface light source device and a liquid cristal panel. The surface light source device is according to the invention and the liquid crystal panel is provided above the light-emitting surface of the light guide plate in the surface light source device.
An inexpensive liquid crystal display device that is free of chrominance or luminance unevenness and has superior display characteristics is obtained by disposing a liquid crystal panel above the surface light source device according to the invention.